The use of Integrated Circuit (IC) cards has been going on for several years in multiple applications. These applications range, in terms of complexity, from simple token-wise payment applications in private environments to complex and intrinsically secure electronic banking applications including powerful encryption and decryption mechanisms. The interaction of the IC in the card with the reader is made through a set of metallic contacts on the surface of the card.
The public interest in many of today's applications of IC cards is greatly increased if the operation of the card does not require a mechanical interaction of the user with the reading device, thus allowing a much faster operation and an increased roughness of the reader that reflects in an increase in the reliability of the system. This is one of the main reasons for the development, which continues today with new standards being defined and tested, of the contactless operation of IC cards and electronic labels or tags.
There has been some effort put in the development of the electromagnetic sensor to be used in the contactless operation of the cards, but the outcome of this development has been the design of the electromagnetic sensor architectures that are not optimally using the available space within the card or tag.
In low frequency applications, where the coupling of the IC chip to the contactless reader is made via an inductive coupling, the most common solution has been the use of multiple-turn coils (see (2) in FIG. 10), which imply quite a complicated manufacturing process because of the fact that the two ends of the coils are located in different sides with respect to the coil windings (see bridge (3) in FIG. 10). Other simpler solutions such as (1) are convenient in terms of manufacturing simplicity, but feature a poor in performance.
In applications at higher frequencies, the minimum size of the electromagnetic sensor is determined by the wavelength at the operation frequency, and this fact implies that there is a clear compromise between the performance of the electromagnetic sensor and its size. This compromise has been solved with a limitation of the electromagnetic sensor size to values that are suitable for the final product, at the expense of obtaining electromagnetic sensor performances that are far from optimal.
The set of geometries named Space-Filling Curves (hereinafter SFC) were described in the patent publication WO 01/54225 wherein said space-filling curve was defined as a curve composed by at least ten connected straight segments, wherein said segments are smaller than a tenth of the operating free-space wave length and they are spatially arranged in such a way that none of said adjacent and connected segments form another longer straight segment, wherein non of said segments intersect to each other except optionally at the tips of the curve, wherein the corners formed by each pair of said adjacent segments can be optionally rounded or smoothed otherwise, and wherein the curve can be optionally periodic along a fixed straight direction of space if and only if the period is defined by a non-periodic curve composed by at least ten connected segments and no pair of said adjacent and connected segments define a straight longer segment.
In said document the space-filling curve features a box-counting dimension larger than one, being said box-counting dimension computed as the slope of the straight portion of a log-log graph, wherein such a straight portion is substantially defined as a straight segment over at least an octave of scales on the horizontal axes of the log-log graph.